Protection circuit for multiple phase power system

ABSTRACT

A protection circuit for three-phase power systems incorporates overcurrent protection along with phase loss and phase imbalance by means of common current sensing transformers. The phase loss and phase imbalance are determined by a pairwise comparison algorithm.

This is a continuation of U.S. application Ser. No. 07/471,336, filedJan. 29, 1990 now abandoned.

BACKGROUND OF THE INVENTION

When electric motors are used within three-phase power distributionsystems, the motor can become over-heated upon the occurrence of acondition commonly known as "single-phasing". The single-phasing occurswhen a fuse in one phase operates to isolate the phase or the wiringwithin one of the phases becomes other-wise disconnected. The remainingtwo phases continue to feed the motor causing the motor to overheat.Although the circuit current increases during the overheatingconditions, most circuit interruption devices are not sufficientlysensitive to respond to the increased current resulting in eventualdamage to the motor.

Most devices employed to interrupt circuit current upon the occasion ofa single-phasing condition require separate sensing devices to measurethe voltage across a set of fuses or to measure the current imbalancewithin the motor itself.

U.S. Pat. No. 4,060,843 discloses separate sensing means coupled tothree-phase power lines for determining the occurrence ofsingle-phasing. U.S. Pat. No. 4,021,703 describes the use of a DC rippledetecting device capable of being utilized on a three-phase circuit todetect phase imbalance. U.S. Pat. No. 4,837,654 teaches the use ofsignal amplifying transistors connected in a serial amplificationconfiguration to detect single-phasing.

One purpose of the instant invention is to describe a circuit capable ofdetecting both phase loss and phase imbalance without requiring separatesensing devices apart from the current transformers used withinelectronic overcurrent circuit interruption devices.

SUMMARY OF THE INVENTION

The invention comprises a phase loss and phase imbalance detectioncircuit employing a plurality of ground referenced operationalamplifiers connected with current transformers to provide a trip signaloutput when the phase loss or phase imbalance is detected. Theoperational amplifiers are arranged in accordance with a pairwisecomparison algorithm and can be used independently from or incombination with standard electronic overcurrent circuit interruptiondevices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a combined overcurrent, phaseimbalance and phase loss protection circuit according to the invention;

FIG. 2 ia diagrammatic representation of a simplified circuit fordetecting phase imbalance and phase loss; and

FIG. 3 is a schematic representation of a circuit interruption circuitincluding an add-on modular phase loss and phase imbalance accessory inaccordance with an alternative embodiment of the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An integrated protection circuit 9 is shown in FIG. 1 and includes aphase loss and imbalance detection circuit 10 connected with a powersupply and trip circuit 11 and with a signal conditioning circuit 12.The signal conditioning circuit is connected with a three-phase powersystem, one conductor of which is represented at 13, by means of acurrent transformer 14 that includes a primary winding 15, core 16, andsecondary winding 17. There are three such phase conductors and currenttransformers although only one phase conductor is shown herein forpurposes of illustration. The secondary winding is connected with abridge rectifier 18 consisting of diodes D₁ -D₄ and from there to aburden resistor R₁ to generate a DC voltage proportional to the currentthrough the first phase conductor. Similar bridge rectifiers 19, 20 anddiodes D₅ -D₁₂ connect with separate burden resistors R₂ and R₃ anddiode D₁₃ to provide a voltage indication of the current through theremaining two phase conductors (not shown). The voltage generated acrossthe burden resistors is transmitted to the power supply and trip circuit11 over conductors 21-23. The trip circuit is similar to that describedwithin U.S. Pat. 4,589,052 entitled "Digital I² T Pickup, Time bands andTiming Control Circuits for Static Trip Circuit Breakers", which Patentis incorporated herein for purposes of reference. Upon the occurrence ofan overcurrent condition in any of the three-phase conductors, a tripsignal is generated and the circuit is interrupted in accordance withthe teachings of the aforementioned United States Patent. The voltagegenerated across the burden resistors is also applied to the inputs ofthree operational amplifiers 24-26 in the following manner. The voltagegenerated across R₁ is applied to the negative input terminal ofamplifier 24 through resistor R₄ and directly to the positive inputterminal of amplifier 26. The voltage appearing across R₂ is directlyapplied to the positive input terminal of amplifier 24 and to thenegative input terminal of amplifier 25 through resistor R₅. The voltageappearing across R₃ is directly applied to the positive input terminalof amplifier 25 and to the negative input terminal of amplifier 26through resistor R₆. The other side of resistors R₁ -R₃ is connectedwith the power supply and trip circuit 11 as well as with systemsground. Integration capacitors C₁ -C₃ are respectively connected acrossthe negative inputs of the operational amplifiers 24-26 and theiroutputs as indicated. The negative input to operational amplifier 24 isconnected through resistor R₇ and R₈ to the negative input ofoperational amplifier 25 and through resistor R₉ to the negative inputof operational amplifier 26. The negative inputs of the three amplifiersare commonly connected to one stage of a voltage divider consisting ofresistors R₁₁ -R₁₃ by means of conductor 29. The outputs of the threeoperational amplifiers are connected in common through diodes D₁₄ -D₁₆and conductor 30 to the positive input terminal of a comparator 27 andcommonly connected to ground through resistor R₁₀. The negative input tocomparator 27 is connected to a second stage of the voltage divider R₁₁-R₁₃ to provide a threshold comparison voltage to the comparator. Aresistor R₁₄ is connected between the positive input to the comparatorand the comparator output which connects over conductor 28 to the powersupply and trip circuit 11 to provide a trip signal thereto upon theoccurrence of a phase loss on phase imbalance condition.

Phase imbalance and phase loss are determined within phase imbalance andthe phase loss circuit 10 by means of the following simple pairwisecomparison algorithm:

    PhLoss/Imbalance=B<(A-K)or A<(C-K)or C<(B-K)

where A, B and C are the three phase currents and

K is minimum imbalance level, or threshold.

The phase currents A, B, C are represented by the voltages generatedacross burden resistors R₁, R₂ and R₃ described earlier with respect tothe signal conditioning circuit 12. The K factor in the expression forthe phase loss algorithm constitutes a setpoint or threshold to preventspurious and nuisance tripping caused by harmonic variations of atemporary nature unless the imbalance exceeds the setpoint by tenpercent, for example. The current in phase B is compared to the currentin phase A within the operational amplifier 24 such that the result ofthe comparison is directed over line 30 to the positive input tocomparator 27 which is compared to the threshold value applied to thenegative input by the second stage of the voltage divider. In the eventthat the current in phase B is less than that of phase A by the amountof the threshold value K, a trip signal is generated at the output ofthe comparator and operates the trip circuit via conductor 28. Thecurrent in phase C is compared to the current in phase B withinoperational amplifier 25 and when the current in phase C is less thanthat of phase B, a similar trip signal is generated. The current inphase A is compared to the current in phase C within operationalamplifier 26 and a similar trip signal is outputted when the current inphase A is less than that of phase C.

A detailed arrangement of the phase loss and phase imbalance circuit 32is depicted in FIG. 2 wherein a threshold voltage is applied at T₁ andthe voltage representing phase A is applied at T₂ with the voltagerepresenting phase B applied to T₃. In this arrangement the K factorwhich represents a setpoint or threshold voltage value is applied at thenegative input to the ground-referenced operational amplifier 33,configured as an integrator, with the integrating capacitor C₁connecting between the output and the negative input terminal. ResistorR₁ represents the voltage value of the K factor whereas the combinationof the integrating capacitor C₁, less the voltage developed across theinput resistor R₂, sets the level of imbalance and the short timeintegration respectively. If desired, both the integrating capacitor andthe input resistor can be made variable to set the values over a widerange of operating conditions. RMS or peak detection could also be usedto determine the comparison rather than the averaging method determinedby means of the operational amplifier 33. In the event that phase A andphase B currents are equal in amplitude, the average signal into theoperational amplifier is zero, and the K factor assures a net negativeinput to the operational amplifier such that the output to the diode D₁remains at ground value via resistor R₃. When the net voltage value ofphase A exceeds the average of the phase B voltage plus the K factor,the output ramps in the positive direction and provides an input to thesecond comparator 34 where it is compared to a four volt referencesupplied to the negative terminal. If it exceeds the four volt referencevoltage, a trip signal is outputted onto the trip line 28.

A combined overcurrent protection, phase loss and phase imbalancecircuit 59 is depicted in FIG. 3 wherein the phase loss and phaseimbalance circuit 48 comprises a stand-alone module which is pluggedinto the signal conditioning and power supply circuit 45 and theovercurrent trip circuit 50. The signal conditioning and power supplycircuit 45 as well as the trip circuit 50 are all included within thecircuit breaker trip unit described in U.S. Pat. No. 4,754,247 entitled"Molded Case Circuit Breaker Accessory Enclosure", which Patent isincorporated herein for purposes of reference. In the circuit breakerdescribed in this Patent, the trip unit 60 and the actuator 57 arearranged within the circuit breaker case and are accessible through anaccessory cover. It is contemplated that the phase loss and phaseimbalance circuit 4 could be arranged in a separate recess within thecircuit breaker cover and provided as an add-on accessory. In thisarrangement, the three phases of a three-phase power circuit comprisingconductors 36, 37 and 38 are protected by means of the circuit breakercontacts represented as switches S₁ -S₃ within each of the separatephases. Current transformers 39-41 connect with the signal conditioningand power supply 45 over conductors 42-44 to provide signals to the tripcircuit 50 over conductors 51, 52 for overcurrent determination. Thesame representative voltage signals are inputted to the phase loss andphase imbalance circuit 48 over conductors 46 and 47. When a phase lossor imbalance condition is ascertained, an output signal is provided tothe trip circuit 50 over conductor 49. The output of the overcurrentdetermination from the trip circuit is inputted over conductor 53 to ORGATE 55. The output from the trip unit relative to the presence of aphase loss or phase imbalance condition is inputted to the OR GATE overconductor 54. Upon the occurrence of either an overcurrent condition ora phase loss or a phase imbalance condition, a trip signal is appliedover conductor 56 to the actuator unit 57. The actuator then interruptsthe current through all three conductors 36-38 by means of the operativeconnection between the actuator and switches S₁ -S₃ as indicated by thecontrol line 58.

A simple phase loss and phase imbalance circuit has herein beendescribed as a stand-alone circuit, an integral part of an overcurrentprotection circuit as well as in the form of an add-on accessory toexisting circuit breakers. The phase loss and phase imbalance circuitimplements an algorithm that determines the occurrence of a phase lossor phase imbalance condition and outputs a trip signal to interrupt allthree phases of a protected three-phase power supply.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:
 1. A circuit interrupter including phaseloss, phase imbalance and overcurrent circuit protection comprising:aplurality of current transformers connecting with corresponding phasesof a multi-phase power system, each of said current transformersproviding a value representative of current flow through each of saidcorresponding phases of said multi-phase power system; a plurality ofrectifiers and resistors connecting with said current transformers forgenerating DC voltage representations of circuit current through each ofsaid phases; a trip circuit connected with said current transformers forproviding operating power to said trip circuit and with said resistorsfor determining overcurrent conditions through said phases andinterrupting circuit current through said phases when said overcurrentconditions persist for a predetermined time; a plurality of amplifiersinterconnected together and connected with said resistors and said tripcircuit to determine phase imbalance between said phases and input atrip signal to said trip unit upon occurrence of said phase imbalanceabove a predetermined threshold; and a trip actuator unit connectingwith said trip circuit and with a plurality of separable contacts tointerrupt said circuit current upon occurrence of said overcurrentcondition or said phase imbalance, said trip circuit, said actuator,said resistors, said amplifiers and said rectifiers being collectivelycontained within a common enclosure.
 2. The circuit interrupter of claim1 including a comparator connecting between said amplifiers and saidtrip circuit for determining whether said phase imbalance exceeds saidthreshold.
 3. The circuit interrupter of claim 2 wherein said amplifiershave corresponding outputs connected in common with one input to saidcomparator.
 4. The circuit interrupter of claim 3 wherein saidamplifiers have corresponding inputs connected in common with each otherand with another input on said comparator.
 5. The circuit interrupter ofclaim 4 wherein said inputs are connected with another input through avoltage divider network.
 6. The circuit of claim 5 wherein said oneinput connects with said comparator output through a feedback resistor.7. The circuit interrupter of claim 1 wherein aid resistors, saidamplifiers and said rectifiers are removably arranged within said commonenclosure.
 8. The circuit interrupter of claim 1 wherein said tripcircuit connects with said actuator through an OR gate.
 9. The circuitinterrupter of claim 1 wherein said amplifiers include acommonly-connected output, said commonly-connected output beingconnected to one side of a summing resistor for providing a voltagerepresentation of said phases and said commonly-connected input beingconnected with a voltage divider for providing said threshold.
 10. Animproved circuit interrupter of the type consisting of a case and acover, said case including means for interrupting circuit currentthrough a protected circuit, said cover including recess means foraccepting an accessory device, wherein the improvement comprises:anovercurrent protection tip circuit for determining overcurrentconditions within three phases of a multi-phase power circuit andactivating said interrupter means when said overcurrent conditionpersists for a predetermined period of time; a phase imbalance accessorydevice within said recess, said device including circuit means fordetermining a phase imbalance within said multi-base power circuit andproviding an interrupt signal to said interruption means to interruptsaid circuit current.
 11. The circuit interrupter of claim 10 includinga comparator connected with said overcurrent protection trip circuit andsaid phase imbalance circuit means to output a trip signal to saidinterrupter means when either an overcurrent condition or a phaseimbalance condition is detected on aid multi-phase power circuit. 12.The circuit of claim 10 including a current transformer within each ofsaid three phases for sampling circuit current and contacts within eachof said phases, said contact responding to said interruption means tointerrupt said circuit current.
 13. The circuit of claim 10 wherein saidphase imbalance protection device is arranged within a module removablyconnectable with said current transformers and said overcurrentprotection circuit.